Galvanized steel sheets or aluminized steel sheets have been widely used as steel sheets for automobiles, household electric appliances, and building materials with their surfaces subjected to chromate treatment using treatment liquids containing hexavalent chromium as the major component to improve corrosion resistance (white rust resistance and red rust resistance).
Also, surface-treated galvanized steel sheets are often used as components for automobiles, household electric appliances, office automation equipment, etc. When used as components such as motor cases, particularly, they are subjected to press forming such as drawing. To improve the press formability of surface-treated galvanized steel sheets, it is effective to apply a lubricant or to provide the surface-treatment coating with lubricity. In some cases, however, continuous high-speed press forming is carried out in which not less than 100 pressed products are produced per minute using, for example, a transfer press. In such continuous high-speed press forming, decreased lubricity due to increased die and lubricant temperature results in galling between a press die and surface-treated steel sheets, thus degrading their appearance after the press forming.
In addition, a step of ironing steel sheets in continuous high-speed press forming involves a severe pressing environment in which sliding movement between the die and the surface-treated steel sheets inevitably causes part of the surface-treatment coating and the plating to peel off and leave peeling scraps. In such a severe pressing environment, peeling scraps accumulate in the lubricant and readhere to the subsequent works. This lowers productivity because cleaning is needed in the final step and degrades the appearance of the works after the press forming because of darkening in the surfaces of the works (surface darkening), which is not readily eliminated by cleaning. Forming a thinner coating to reduce peeling scraps, on the other hand, decreases flat-portion corrosion resistance before press forming and also decreases corrosion resistance after press forming because the coating peels off during press forming.
Surface-treated galvanized steel sheets subjected to conventional chromate treatment are subjected to press forming with a lubricant applied thereto because the chromate coating itself has no lubricity. The chromate coating, however, maintains its superior corrosion resistance even after press forming in a severe pressing environment, such as continuous high-speed press forming, while preventing considerable degradation of the appearance after the press forming because even an ultrathin chromate coating provides superior rust resistance.
In other cases, surface-treated galvanized steel sheets are subjected to press forming with an organic lubricant coating formed as a second layer after chromate treatment to provide lubricity so that the steps of applying a lubricant and degreasing are not required. A surface-treated galvanized steel sheet with an organic lubricant coating has superior performance in terms of appearance and corrosion resistance after press forming if conditions such as continuity, forming rate, and ironing are mild. The appearance and corrosion resistance after press forming, however, are considerably degraded in a severe pressing environment as described above even if the forming is performed after lubrication. This is probably because peelings tend to adhere to the works and the die during the press forming. The surfaces of the works tend to be darkened, and the peelings adhering to the die damage the surfaces of the subsequent works, thus decreasing their corrosion resistance.
Recently, as a measure against global environmental problems, surface-treated steel sheets with surface-treatment coatings free of hexavalent chromium, an environmentally controlled substance, have increasingly been used instead of conventional chromate treatment, and the following chromate-free treatment methods have been proposed to inhibit white zinc rust on galvanized steel sheets.
For example, (1) a method based on the passivation of molybdic acid or tungstic acid, which belongs to the same group as chromic acid, namely, the IVA group; (2) a method using a metal salt of a transition metal such as titanium, zirconium, vanadium, manganese, nickel, or cobalt or of a rare-earth element such as lanthanum or cerium; (3) a method based on a chelating agent such as a polyphenol carboxylic acid, e.g., tannic acid, or a compound containing sulfur or nitrogen; (4) a method in which a polysiloxane coating is formed using a silane coupling agent; and combinations thereof are available.
Specifically, the following methods are available:
(1) a method in which a coating is formed using a treatment liquid containing an organic resin such as a polyvinyl phenol derivative together with, for example, an acid component, a silane coupling agent, and a vanadium compound (for example, Patent Documents 1, 2, and 12);
(2) a method in which a coating containing a water-based resin, a thiocarbonyl group, a vanadic acid compound, and phosphoric acid is formed (for example, Patent Document 3);
(3) a method in which a coating is formed using a treatment liquid containing a metal compound such as a titanium compound together with a fluoride, an inorganic acid such as a phosphoric acid compound, and an organic acid (for example, Patent Documents 4, 5, 8, 9, 10, 11, and 13); and
(4) a method in which a compound coating of a rare-earth element such as cerium, lanthanum, or yttrium and titanium or zirconium is formed so that an oxide layer and a hydroxide layer are concentrated on the plating interface side and the surface side, respectively, in the coating (Patent Document 6), and a method in which a compound coating of cerium oxide and silicon oxide is formed (Patent Document 7).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-13252    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-181860    Patent Document 3: Japanese Patent No. 3549455    Patent Document 4: Japanese Patent No. 3302677    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2002-105658    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2001-234358    Patent Document 7: Japanese Patent No. 3596665    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2004-183015    Patent Document 9: Japanese Unexamined Patent Application Publication No. 2003-171778    Patent Document 10: Japanese Unexamined Patent Application Publication No. 2001-271175    Patent Document 11: Japanese Unexamined Patent Application Publication No. 2006-213958    Patent Document 12: Japanese Unexamined Patent Application Publication No. 2004-263252    Patent Document 13: Japanese Unexamined Patent Application Publication No. 2005-48199
The above coatings in the conventional art are intended to inhibit white zinc rust through the combined addition of organic and inorganic components. The methods (1) and (2) take the approach of achieving corrosion resistance primarily by adding an organic resin. In particular, organic-resin-rich coating compositions are superior in flat-portion corrosion resistance, although they are extremely inferior in appearance and corrosion resistance after continuous high-speed press forming because, as described above, peelings tend to adhere to the works and the die during the press forming.
The methods (3) and (4), on the other hand, propose a single inorganic coating free of organic components. A compound coating of metal oxide/hydroxide, however, is disadvantageous for continuous high-speed press forming because its thickness must be increased to achieve sufficient corrosion resistance, and the increased thickness tends to result in coating unevenness and coloring. In addition, such techniques cannot provide sufficient corrosion resistance for thin coatings because they are designed without considering appearance and corrosion resistance after press forming, particularly those after continuous high-speed press forming, thus failing to achieve both superior corrosion resistance and appearance after press forming.
Thus, although the above conventional techniques provide some corrosion resistance, numerous types of organic-resin-based coatings and thick coatings that have so far been proposed are unsuitable for continuous high-speed press forming and cannot achieve both superior flat-portion corrosion resistance and appearance and corrosion resistance after continuous high-speed press forming. As for the formation of an inorganic coating, no technique has been proposed that provides sufficient flat-portion corrosion resistance, blackening resistance, and appearance and corrosion resistance after continuous high-speed press forming.
Accordingly, an object of the present invention is to solve the above problems in the conventional art and provide a surface-treated galvanized steel sheet with a coating free of hexavalent chromium that has superior flat-portion corrosion resistance, blackening resistance, and appearance and corrosion resistance after continuous high-speed press forming.
Another object of the present invention is to provide an aqueous surface-treatment liquid with which such a surface-treated galvanized steel sheet can be stably produced and a method for producing a surface-treated galvanized steel sheet using the aqueous surface-treatment liquid.